Frequency division multiplexing (FDM) is a technology for transmitting different data sets within each of multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within a carrier—a unique frequency range that is modulated by data being transmitted.
Orthogonal frequency division multiplexing (OFDM) is a spread spectrum technique that distributes each data set of the different data sets over a large number of carriers that are spaced apart at predetermined frequencies. This spacing provides the “orthogonality” in this technique, which allows for demodulators that are responsive only to frequencies relating to a signal data set. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. OFDM is advantageous because in a typical terrestrial broadcasting scenario there are multipath-channels—transmitted signals arrive at a receiver using various paths of different length. Since multiple versions of a signal interfere one with another it becomes difficult to extract data being transmitted.
For amplifying an OFDM signal, an amplifier must support a range of pulse amplitudes from a first level of amplitude through a peak amplitude. Though support for peak amplitude is a requirement of OFDM standards, OFDM peak pulses come with such infrequency that designing an amplifier to support them, though required, increases the power consumption of the PA and adds a level of complexity and cost that is undesirable.
For example, it is known to improve power consumption of PA's by varying supply voltage with a DC to DC converter to be proportional to the amplitude of the transmitted signal. Lower collector voltages are used to achieve lower output powers and higher collector voltages are used to achieve higher voltage values. Assuming high efficiencies in the regulator, very low power consumption is realized at low RF output powers. Unfortunately, this approach requires relatively large regulator components capable of delivering >700 mA with a very clean output spectrum. In general, the size and conversion efficiency of this regulator type is more problematic at high current due to the dropping losses in the pass transistors. Ultimately, The approach suffers from cost and size issues.
The regulator approach is Prior Art and is an effective way of increasing the PA efficiency, by varying the collector or drain voltage on the amplifying transistor and changing the load line of the PA. Linearity requirements, however, force the gain/phase response to be linear with voltage change, or that pre-distortion is applied.
Other variants on this theme attempt to use a very fast, envelope tracking power supply on the collector in combination with a variable base supply. The modulation amplitude is realized by varying the power supply voltage while the phase information is injected onto the RF signal. Envelope tracking requires an even more complex power supply than the DC to DC converter approach and has yet to demonstrated in a practical fashion.
It would be advantageous to provide a method and apparatus to improve the power output, efficiency, and distortion of an OFDM power amplifier without significantly increasing the power supply complexity, or needing a second voltage supply. Advantageously, improving these attributes is beneficial in WLAN systems in order to provide users with better data transmission range, longer intervals between battery charging, and more generally lower power consumption.